The invention relates to a four-cycle engine which is preferably used in a small jet boat or the like, and also to a small jet boat on which such a four-cycle engine is mounted.
A small jet boat is a recreational watercraft for one or two riders which glides over water during use.
A jet boat uses a water jet pump or a screw propeller as the propulsion means. Conventionally, a two-cycle engine is mounted as an engine for driving the propulsion means because a two-cycle engine is small in size and weight and has a lubrication system which is suitable for a small jet boat. Specifically, in a two-cycle engine, lubrication is done by the drysump system in which lubricating oil is not accumulated in a crankcase. A small jet boat is a vehicle characterized in that the boat often overturns (or capsizes), the operator or another person raises the boat each time when it overturns, and the operator then desirably continues to enjoy the gliding action over water. Consequently, it is preferred to use a drysump system in which, even when the boat overturns, no lubricating oil enters from the crankcase into a combustion chamber.
Such an engine is mounted together with suction and exhaust devices in a space surrounded by so-called shell plating of a hull, such as a bottom hull, a hood, and a seat. This configuration is adopted because of the following reasons. If the engine, etc. are protruding below the bottom hull, the boat cannot smoothly glide over the water. If the engine, etc. are protruding above or to the side of the hood or the seat, water can be directly splashed on the engine, etc., thereby impeding normal combustion and causing the engine, etc. to easily rust, and making it difficult for a person to go on board the boat. In a small jet boat, however, the space (i.e., the engine compartment) for containing an engine is usually restricted in size to a considerably small volume because of the following reasons. If the engine compartment has a large volume, the small jet boat itself is large in size so that operation properties peculiar to such a boat, such as speed and turning characteristics are apt to be impaired. In a small jet boat, generally, a seat has a trapezoidal section shape which is extended in the lower side, so that a person easily sits astride the seat. If the seat is enlarged in width, it is difficult for a person to sit astride the seat.
Because of these reasons, conventionally, a two-cycle engine is used as a propulsion engine for a small jet boat. Since a two-cycle engine does not require an oil pan, a valve, a valve operating mechanism, and the like, the engine is compact in size. Furthermore, a two-cycle engine provides a high power output because of its high explosion frequency. Consequently, a two-cycle engine is very suitable for a small jet boat.
In contrast, a normal four-cycle engine employs a wetsump lubrication system in which lubricating oil is accumulated in a crankcase. If a boat overturns with a normal four-cycle engine, therefore, lubricating oil enters a combustion chamber via a gap between a cylinder and a piston. When such entrance of lubricating oil into a combustion chamber occurs, exhaust gases contain white smoke and, in many cases, the engine stalls and is hard to restart. In the prior art, therefore, a four-cycle engine is not used in a small jet boat.
An example of a small jet boat on which a two-cycle engine is mounted is disclosed in, for example, Japanese Utility Model Publication (Kokai) No. HEI 3-61152.
In considering noise reduction and the prevention of air pollution, recently, the availability of a four-cycle engine for a small jet boat or the like has begun to be desired.
In order to use a four-cycle engine in a small jet boat or the like, however, it is required to solve the above-discussed problems concerning lubricating oil in the case of turnover of a small jet boat or the like and concerning a small housing space.
If a prior art four-cycle engine of the wetsump system is used in a small jet boat, lubricating oil undesirably enters from a crankcase into a combustion chamber in the case of turnover. A four-cycle engine of the drysump system maybe employed to meet this problem, however, the engine is considerably complicated in structure because of the following reason. In the drysump system, lubricating oil is not accumulated in a crankcase. Consequently, it is necessary to separately provide an oil tank and a lubricating oil recovery pump which sucks lubricating oil from the crankcase and feeds it to the tank. Furthermore, a lubrication system (a filter, a pressure pump, pipes, etc.) is configured for supplying lubricating oil from the tank to various portions. As seen from the above, unlike a two-cycle engine in which a drysump system is inevitably employed because of the inherent feature that a sucked air-fuel mixture is pressurized in a crankcase, the employment of a drysump system a four-cycle engine, which has more complicated structure, is very disadvantageous from a practical view point.
Furthermore, a four-cycle engine is larger in size than a two-cycle engine of the same output power. When a four-cycle engine is to be mounted as a propulsion engine on a small jet boat, therefore, there arise problems regarding the location of the hull where the engine is to be placed and regarding the manner of mounting the engine. Such problems cannot be easily solved because of the reasons discussed below.
(1) In the same manner as the case where a two-cycle engine is mounted, the engine compartment of a small jet boat is restricted in size. In a small jet boat of the type in which a person sits astride a seat, particularly, the width of the seat is limited by body dimensions of a rider and hence also the engine compartment under the seat has a very small width.
(2) As described above, a shape of the engine compartment is similar to that of said seat and is a trapezoidal section shape which is extended in the lower side thereof.
In addition to the fact that the top part of a four-cycle engine is larger in size than that of a two-cycle engine, there is a restriction that suction devices such as an intake silencer and a carburetor must be placed in positional relationships (same as those in the case of a two-cycle engine) with respect to the engine in which certain conditions are satisfied and the devices are not separated from the engine by a large distance. Therefore, the suction devices of a four-cycle engine, must be compactly housed in the engine compartment adjacent to the engine.
A four-cycle engine has a large size because of the reasons including: first, intake and exhaust valves (poppet valves) are provided and hence a large space is inevitably formed above a cylinder; second, a space for an oil pan is required in a wetsump lubrication system in which lubricating oil is accumulated in a crankcase in the same manner as a usual four-cycle engine; and third, a four-cycle engine is lower in explosion frequency than a two-cycle engine and hence the number of cylinders and the displacement must be increased in order to attain the same output power. Suction devices must be connected to an engine in a substantially integrated manner because of the following reasons. For efficiency, such devices (particularly, a carburetor) should be placed above a cylinder so that fuel particles smoothly enter the combustion chamber without going against gravity. In order to reduce the suction resistance, the devices should be disposed in the vicinity of a cylinder.
(3) Since a four-cycle engine is larger also in weight than a two-cycle engine, the weight distribution in a small jet boat must be considered with particular attention. In order to attain balance and stability in a small jet boat, an engine must be placed at a desirable position in the longitudinal and transverse directions of the hull. Generally, a space in which an engine can be mounted is narrow in the width (transverse) direction, and the location of an engine therefore is particularly restricted in that direction. Therefore, it is not easy to properly position an engine so as to balance the weight of the engine in the transverse directions of the hull.
(4) In propulsion means for a small jet boat, such as a water jet pump or a screw propeller, the drive shaft is disposed in the vicinity of the bottom of the hull. In contrast, since a four-cycle engine requires an oil pan, the crank shaft of the engine is at a position higher than that of a two-cycle engine (an upward position separated from the lowest portion of the engine). Depending on the quantity of lubricating oil which is to be stored in the oil pan, and the shapes of the oil pan and the crankcase, this may require that the crank shaft is disposed at so high position that the crankshaft can hardly be connected with the drive shaft at the appropriate point, and/or that the drive shaft must be unreasonably angled.